The present invention relates to a color electrophotographic method and a method for stably forming color images of high quality by digital signal processing of image information.
As a method for color image formation by electrophotographic process, CPC electrophotographic method according to so-called analogue processing has been known which comprises repeating necessary times the steps of irradiating an original image with light, exposing a charged photoreceptor through a color separation filter to the light reflected from or transmitted through the original image, thereby to form an electrostatic latent image on the photoreceptor, then developing the latent image with color toner, whereby toner image is directly formed on the photoreceptor. This CPC process is said to be able to easily produce images superior in resolving power to so-called PPC process which comprises transferring toner images.
The inventors have paid their attention to the fact that titanium dioxide is good in so-called halftone image reproducibility and excellent in whiteness and hiding power as photoconductive material of photoreceptor in the electrophotographic method according to the above-mentioned CPC process and have conducted research in an attempt to produce so-called pictorial color images excellent in halftone reproducibility, graininess and resolving power comparable to those of silver halide photographic print by electrophotographic method, namely, by forming electrostatic latent image using the titanium dioxide photoreceptor and then developing this latent image with a liquid developer prepared by dispersing toner particles in an electrically insulating liquid which gives image of superior graininess. As a result, it has been recognized that quality of the resulting color image is considerably improved, but there are still many problems to be solved.
That is, (1) in case of the analogue process, as is clear from the gradation characteristic of this process as shown in FIG. 2, original image density vs output image density characteristics becomes nonlinear to bring about distortion in color reproduction. In this respect, further explanation will be made based on the tone reproduction characteristic curves in analogue process as shown in FIG. 2 (liquid developing was carried out with a liquid developer using a photosensitive paper applied with titanium dioxide as a photoconductor). The first quadrant shows relation between original image density and output image density. The second quadrant shows relation between surface potential of photoreceptor in development and output image density, that is indicates characteristics of toner used in development. The third quadrant shows relation between exposure and surface potential of photoreceptor, that is indicates light decay characteristics of photoreceptor. The fourth quadrant shows relation between original image density and exposure and this relation is linear in analogue process, because that exposure is fundamentally carried out with light quantity in proportion to reflectance or transmittance of original image. Since characteristics of photoreceptor in the third quadrant and characteristics of toner in the second quadrant are both nonlinear and besides are not in the relation to compensate the mutual nonlinearity, the relation between the original image density and output image density is not linear, but shows distortion. Distortion is great especially in higher density area and image of higher density area tends to be indistinguishable. That is, when a color image is obtained by superposing three color separation images of yellow, magenta and cyan, even in the case of titanium dioxide photosensitive paper which is wide in tone reproduction region, reproduction is as shown in FIG. 2 and there is a limit in reproduction of original. In case of analogue system, even if exposure is changed so as to correct distortion of color reproduction caused by nonlinear effect, only the quantity of light changes depending on distortion of decay characteristics. Therefore, even if, for example, "red" of middle color density is made pale in order to make it to favorite tone with keeping expression of pale and deep portions of "red" as they are, since the operation is only the prolongation of exposing time, the pale "red" becomes paler and the deep "red" also becomes pale, resulting in loss of balance as a whole. Thus, in analogue system, it has been substantially impossible to correct only the desired color without losing balance with others.
(2) Furthermore, photoreceptor is susceptible to influence by variation of environmental conditions such as temperature and humidity and due to the variation, charging characteristics and photosensitivity characteristics are greatly changed and stability of image is apt to be damaged. Therefore, proposal to provide severe control mechanism of environmental condition or to use various moistureproofness imparting agents have been made, but the above-mentioned influences have not yet been fully avoided. Furthermore, in color electrophotographic method according to so-called CPC process by which toner images are superposed on photoreceptor, with reference to the second and the following colors, exposure is carried out with light which transmits the layer of the toner image which has been developed on a photoreceptor at the previous image forming step and hence, there occurs difference in quantity of light which reaches the photoreceptor due to so-called light screening effect owing to toners between the portions of photoreceptor where the toner image is present and no toner image is present. That is, quantity of light which reaches the portion of photoreceptor where toner image is present is smaller and correspondingly light decay of surface potential is smaller and as a result excessive amount of toner sticks to the photoreceptor to cause turbidity of color. For example, in case cyan is developed as the first color and yellow and magenta are developed as the second and the third colors, since cyan has unwanted absorption in 400-600 nm as shown in FIG. 3, when blue light exposure necessary for development of yellow is crried out over cyan iamge, surface potential is higher about 30V in the portion where cyan image is present than in the portion where no cyan image is present and correspondingly yellow is much developed. This is the same in case of green light exposure for development of magenta. In analogue system according to which photoreceptor is subjected to exposure directly to respective color separation lights, it is impossible to avoid influence of toner image developed on the photoreceptor at the previous image forming step. Therefore, there has been employed such image forming method as superposing the toners in the order of causing less light screening effect of toner. This method is still not satisfactory in color clarity of image.
As opposed to the analogue process, so-called digital image recording process has been known which includes converting an original image to electric signal by photoelectric conversion means, further converting the electric signal to digital signal, subjecting the digital signal to various digital signal processing and carrying out exposure by the means to convert electric signal to light such as laser beam. It is said that according to this digital process, since information of original is obtained as digital electric signal, color correction which cannot be performed by the analogue process can be easily made at the stage of electric signal, and consequently color reproduction can be improved. However, cadmium sulfide, selenium, amorphous silicon, zinc oxide and organic photoconductors which are widely used as electrophotographic photoreceptors give hard tone and are liable to produce binary image of ON and OFF. Therefore, in case of reproduction of halftone using these photoconductors, for this purpose, it is necessary to carry out an image processing by making, for example, a special image processing for halftone reproduction, for example, by dither method. According to the dither method, one matrix comprises a plurality of picture elements adjacent to each other at their length and breadth and threshold values corresponding to respective picture elements are made different in the matirx to carry out conversion to binary image and output as ON, OFF dots. Therefore, in laser exposure method based on the dither method, influence of variation of environmental conditions such as temeprature and humidity on the formation of image is relatively small and besides, since the tone expression is by area modulation system and size of the matrix is much larger than the picture elements, some improvement of tone expression can be seen in the dither method, but considerable reduction of resolving power and graininess than according to analogue process is unavoidable.
Furthermore, for the exposure by laser beam it is desired to use semiconductor laser beam which is easy in control of operation and besides, its emission wavelength is mainly composed of from about 830 nm to 780 nm. However, as is clear from spectral characteristcs of FIG. 3, coloring materials used for toner of developer are not complete in transmission of light of around 780 nm and as shown in FIG. 4, absorption of, for example, about 40% is unavoidable when deposition amount of cyan toner increases. Thus, in the color electrophotographic method according to CPC process of superposing respective color toners, color turbidity is brought about and this effect cannot be ignored.